IMPROVING THE RECORD OF CENOZOIC ATMOSPHERIC CO2 WITH STOMATAL INDEX OF FOSSIL GINKGO LEAVES FROM NORTH AMERICA

A high-resolution record of atmospheric CO₂ concentrations in the past is necessary for understanding the effects of future greenhouse events. The inverse relationship between the stomatal index (SI) of Ginkgo leaf cuticles and atmospheric CO₂ levels, calibrated from greenhouse experiments and herbarium specimens, has been used to compile a record of CO₂ from fossil Ginkgo leaves. Temporal resolution of that record has been limited by availability of suitably preserved leaf cuticles, but useful data can now be gained by joint back-scatter and secondary electron microscopic examinations of fossil impressions. This opens the possibility of a higher resolution record that resolves short-lived greenhouse spikes. For example, impressions of late Paleocene Ginkgo cranei from Almont, South Dakota, close in age to the Paleocene-Eocene Thermal Maximum, had SI of 6.39 ± 0.24 %, which corresponds to 1167 ppmv CO₂ using the 2009 transfer function of Retallack. In contrast, Ginkgo adiantoides from the Oligocene (30 Ma) Lyons flora of Oregon had near-modern SI of 9.6 ± 0.16 %, corresponding to 362 ppmv. A sample of Ginkgo adiantoides from the middle Miocene (16.5 Ma) Musselshell flora of Idaho has SI of 7.23 ± 0.34 %, or 771 ppmv CO₂, reflecting the start of a middle Miocene thermal maximum. This last result highlights the importance of understanding middle Miocene paleoclimate and vegetation as a guide to our future if CO₂ concentrations pass such levels before the year 2100.